Power supply apparatus with soft start control

ABSTRACT

A power supply apparatus may include a plurality of power supply circuits, an individual soft start circuit including a first terminal for outputting a predetermined fixed voltage and a second terminal for outputting a voltage that varies with time, and an individual soft start switch. The individual soft start circuit may be operated when any of the plurality of supply voltages output from the power supply circuits is caused to make a transition individually. The individual soft start switch may connect the power supply circuit with the first terminal or the second terminal of the individual soft start circuit. The plurality of power supply circuits may apply soft start control to the respective supply voltages in accordance with the soft start voltage output from the individual soft start circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of the U.S. patentapplication Ser. No. 11/387,631 filed Mar. 23, 2006, the contents ofwhich are incorporated by reference herein in their entirety, priorityto which is claimed under 35 U.S.C. § 120. U.S. application Ser. No.11/387,631 is a continuation of PCT/JP2004/017414, filed on Nov. 24,2004, the entire contents of which are incorporated herein by reference,and which claims the benefit of the date of the earlier filed JapanesePatent Applications No. JP2003-424455 filed on Dec. 22, 2003 and No.JP2004-326320 filed on Nov. 10, 2004, priority to each of which is alsoclaimed herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply apparatus such as aswitching regulator and, more particularly, to a soft start technologyassociated with the apparatus.

2. Description of the Related Art

FIG. 12 is a block diagram of a power supply apparatus according to therelated art. As illustrated, the power supply apparatus is usuallyprovided with soft start circuits 101 a-101 c for the purpose ofpreventing destruction of switching transistors due to a rush currentoccurring at start-up and preventing overshoot in the waveform of anoutput voltage as it rises.

The soft start circuits 101 a-101 c are configured to gradually increase(soft start) an output voltage by increasing a conduction period (onduty) of the switching transistors over a predetermined period of timesince start-up. For example, on-duty of the switching transistors iscontrolled so that the output voltage is increased in accordance withthe waveform of the charged voltage of capacitors 104 a-104 c of atime-constant circuit.

However, when a plurality of loads (117 a-117 c) requiring differentsupply voltages (Vo1-Vo3) are provided in a single apparatus, there mayarise a need to provide a plurality of power supply circuits 105 a-105 cin the apparatus.

In this case, the power supply apparatus is often subject to on and offcontrol by a host 111 after a sufficient period of time has elapsedsince power from a power supply voltage Vcc indicated in the drawing isturned on (hereinafter, referred to as power-on), for the purpose ofpower saving. Accordingly, such a multi-channel power supply apparatusneeds the soft start circuits 101 a-101 c and the external capacitors104 a-104 c for respective channels, thwarting efforts reduce the numberof components.

“Example of applied circuits” section in non-patent document 1 describes“capacitors SS1, SS2, SS3” which correspond to the external capacitorsdescribed above.

[Non-Patent Document]

Manual of “HDD system power supply BD9786KN”, January 2003, RohmCorporation

SUMMARY OF THE INVENTION

The present invention has been made in view of the aforementionedproblems and relates to a power supply apparatus capable of subjectingeach of a plurality of supply voltages to soft start, while preventingthe number of components from being increased.

At least an embodiment of the present invention may include a pluralityof power supply circuits, an individual soft start circuit including afirst terminal for outputting a predetermined fixed voltage and a secondterminal for outputting a voltage that varies with time, and anindividual soft start switch. The individual soft start circuit may beoperated when any of the plurality of supply voltages output from thepower supply circuits is caused to make a transition individually. Theindividual soft start switch may connect the power supply circuit withthe first terminal or the second terminal of the individual soft startcircuit. The plurality of power supply circuits may apply soft startcontrol to the respective supply voltages in accordance with the softstart voltage output from the individual soft start circuit.

An embodiment of the present invention provides a power supplyapparatus. The power supply apparatus comprises: a plurality of powersupply circuits which supply voltages to respective loads; a common softstart circuit which is operated when the plurality of voltages outputfrom the plurality of power supply circuits are caused to make atransition simultaneously; an individual soft start circuit including afirst terminal for outputting a predetermined fixed voltage and a secondterminal outputting a voltage that varies with time, the individual softstart circuit being operated when any of the plurality of supplyvoltages output from the plurality of power supply circuits is caused tomake a transition individually; a common soft start switch provided foreach of the plurality of power supply circuits, the common soft startswitch connecting the power supply circuit with the common soft startcircuit; an individual soft start switch provided for each of theplurality of power supply circuits, the individual soft start switchconnecting the power supply circuit with the first terminal or thesecond terminal of the individual soft start circuit. The plurality ofpower supply circuits subject the respective supply voltages to softstart control in accordance with the soft start voltage output from thecommon soft start circuit or the individual soft start circuit.

According to this embodiment, by allowing the two soft start circuits tobe shared by the plurality of power supply circuits, the voltages fromthe circuits are concurrently subject to soft start control at, forexample, power-on or power-off. In contrast, when any of the powersupply circuits is singularly turned on or off, the individual softstart circuit is used. With this, the number of components is reduced ascompared with a case where a soft start circuit is provided for each ofthe plurality of power supply circuits.

The plurality of power supply circuits may close the individual softstart switch in accordance with an externally supplied control signaland apply soft start control to the respective supply voltages inaccordance with the soft start voltage output from the individual softstart circuit.

The plurality of power supply circuits may close the common soft startswitch at power-on or power-off and apply soft start control to therespective supply voltages in accordance with the soft start voltageoutput from the common soft start circuit.

The common soft start circuit may comprise: a constant current sourcewhich charges a common soft start capacitor by supplying a constantcurrent thereto; and a transistor which is provided parallel with thecommon soft start capacitor and discharges the common soft startcapacitor in accordance with a timing signal input to the gate of thetransistor.

The individual soft start circuit may comprise: a latch power supplywhich outputs a predetermined voltage; and a digital-to-analogconversion circuit which outputs a voltage of a ramp waveform inaccordance with a digital signal input to a digital input terminal, theoutput voltage at its maximum level being equal to the predeterminedvoltage, wherein the predetermined voltage may be output from a firstterminal and the voltage with a ramp waveform may be output from asecond terminal.

The individual soft start circuit may comprise: a latch power supplywhich outputs a predetermined voltage; an individual soft startcapacitor; a constant current source which charges the individual softstart capacitor by supplying a constant current thereto; and atransistor which is provided parallel with the individual soft startcapacitor and discharges the individual soft start capacitor inaccordance with a timing signal input to the gate of the transistor,wherein the predetermined voltage may be output from a first terminaland a voltage at a node between the individual soft start capacitor andthe constant current source may be output from a second terminal.

The individual soft start circuit may comprise: a latch power supplywhich outputs a predetermined voltage; an individual soft startcapacitor; a constant current source which charges the individual softstart capacitor by supplying a constant current thereto; and atransistor which is provided parallel with the individual soft startcapacitor and discharges the individual soft start capacitor inaccordance with a timing signal input to the gate of the transistor,wherein the predetermined voltage may be output from a first terminaland a voltage at a node between the individual soft start capacitor andthe constant current source may be output from a second terminal.

The individual soft start circuit may comprise: a latch power supplywhich outputs a predetermined voltage; an individual soft startcapacitor; a charge transistor provided between the individual softstart capacitor and the latch power supply; and a discharge transistorwhich is provided parallel with the individual soft start capacitor anddischarges the individual soft start capacitor in accordance with atiming signal input to the gate of the discharge transistor, wherein thepredetermined voltage may be output from a first terminal and a voltageat a node between the individual soft start capacitor and the dischargetransistor may be output from a second terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is a block diagram of a power supply apparatus according to atleast an embodiment of the present invention.

FIGS. 2A through 2C are time charts showing voltage variation atrespective nodes.

FIG. 3 is a block diagram of the power supply apparatus according to atleast an embodiment of the present invention.

FIGS. 4A through 4C are time charts showing a control signal from a hostand voltage variation at respective nodes.

FIGS. 5A through 5C are time charts showing a control signal from a hostand voltage variation at respective nodes.

FIGS. 6A through 6C are time charts showing voltage variation atrespective nodes in the power supply apparatus of FIG. 1.

FIG. 7 is a circuit diagram of a power supply circuit according to atleast an embodiment of the present invention.

FIG. 8 illustrates time waveforms indicating the operation of the powersupply circuit according to at least an embodiment of the presentinvention.

FIG. 9 is a circuit diagram of a common soft start circuit according toat least an embodiment of the present invention.

FIGS. 10A through 10D are circuit diagrams of individual soft startcircuits according to at least an embodiment of the present invention.

FIGS. 11A through 11I are time charts showing the waveforms of controlsignals and open and closed states of switches.

FIG. 12 is a block diagram of a power supply apparatus according to therelated art.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description of at least an embodiment of the presentinvention will be given by referring to the attached drawings. FIG. 1and FIG. 3 are block diagrams of a power supply apparatus according toat least an embodiment of the present invention. FIGS. 2A through 2C aretime charts indicating voltage variation at respective nodes atpower-on. FIGS. 4A through 4C are time charts indicating voltagevariation at respective nodes occurring when a power supply circuit 105b is turned off. FIGS. 5A through 5C are time charts indicating voltagevariation at respective nodes occurring when the power supply circuit105 b in an off state shifts to an on state. FIGS. 6A through 6C aretime charts showing voltage variation at respective nodes occurring atpower-off. FIG. 7 is a circuit diagram of a power supply circuitaccording to at least an embodiment of the present invention. FIG. 8illustrates time waveforms indicating the operation of the circuit. FIG.9 illustrates an example of circuit structure of a common soft startcircuit 103 and a common soft start capacitor 104 according to at leastan embodiment of the present invention. FIGS. 10A through 10D arecircuit diagrams of individual soft start circuits according to at leastan embodiment of the present invention. FIGS. 11A through 11I are timecharts indicating signal waveforms and voltage variation occurring inthe power supply apparatus.

(1) Overall Structure of Power Supply Apparatus

Firstly, the overall structure of the power supply apparatus accordingto at least an embodiment will be described by referring to FIG. 1. FIG.1 is a block diagram illustrating the structure of a power supplyapparatus 1000 according to at least an embodiment of the presentinvention. The power supply apparatus 1000 is provided with three powersupply circuits 105 a-105 c which output supply voltages Vo1-Vo3 todifferent loads 117 a-117 c, respectively.

Referring to FIG. 1, the power supply voltage for the apparatus as awhole is Vcc (V). When power derived from the power supply voltage Vccis turned on, the supply voltages Vo1-Vo3 (V) are output to the loads117 a-117 c, respectively.

The power supply apparatus 1000 includes a voltage reduction resetcircuit 113, an individual soft start circuit 101, a common soft startcircuit 103, a common soft start capacitor 104, power supply circuits105 a-105 c, coils 107 a-107 c, smoothing capacitors 109 a-109 c andswitches SW.

The power supply circuit 105 a is connected to the load 117 a via thecoil 107 a. Similarly, the power supply circuit 105 b and the powersupply circuit 105 c are connected to the load 117 b and the load 117 c,respectively, via the coil 107 b and the coil 107 c, respectively.

An end of the smoothing capacitor 109 a for smoothing the voltage Vo1supplied to the load 117 a is connected to a node between the coil 107 aand the load 117 a. The other end of the smoothing capacitor 109 a isgrounded. Similarly, an end of the smoothing capacitor 109 b forgenerating the voltage Vo2 supplied to the load 117 b is connected to anode between the coil 107 b and the load 117 b. The other end of thesmoothing capacitor 109 b is grounded. Further, an end of the smoothingcapacitor 109 c for generating the voltage Vo3 supplied to the load 117c is connected to a node between the coil 107 c and the load 117 c. Theother end of the smoothing capacitor 109 c is grounded.

Each of the power supply circuits 105 a-105 c is connected to theindividual soft start circuit 101 and the common soft start circuit 103.

The power supply circuit 105 a is connected to switches SW1 a-SW3 a. Byselectively closing the switches SW1 a-SW3 a, the power supply circuit105 a is connected to a first terminal 1011 of the individual soft startcircuit 101, a second terminal 1013 of the individual soft start circuit101 or an output terminal 1014 of the common soft start circuit 103.

The power supply circuit 105 b is connected to switches SW1 b, SW2 b andSW3 b. By selectively closing the switches SW1 b, SW2 b and SW3 b, thepower supply circuit 105 b is connected to the first terminal 1011 ofthe individual soft start circuit 101, the second terminal 1013 of theindividual soft start circuit 101 or the output terminal 1014 of thecommon soft start circuit 103.

The power supply circuit 105 c is connected to switches SW1 c-SW3 c. Byselectively closing the switches SW1 c-SW3 c, the power supply circuit105 c is connected to the first terminal 1011 of the individual softstart circuit 10, the second terminal 1013 of the individual soft startcircuit 101 or the output terminal 1014 of the common soft start circuit103.

The soft start voltage from one of the three systems is switchably inputto each of the power supply circuits 105 a-105 c. Details will bedescribed later. The switches SW1 a-SW1 c are common soft start switchesfor connecting the power supply circuits 105 a-105 c to the common softstart circuit 103.

The switches SW3 a-SW3 c are individual soft start switches that areclosed when the individual soft start circuit 101 is started so as toconnect the power supply circuits 105 a-105 c to the second terminal1013 of the individual soft start circuit 101.

The switches SW2 a-SW2 c are connected to the first terminal 1011 of theindividual soft start circuit 101 and are individual soft start switchesfor fixing the soft start voltage when it has risen.

Hereinafter, these switches SW1 a-SW1 c, SW2 a-SW2 c and SW3 a-SW3 cwill be referred to as switches SW where distinction thereof is notessential.

The common soft start capacitor 104 is a time-constant circuit used inperforming common soft start described later.

The voltage reduction reset circuit 113 is a circuit for transmitting areset signal Vrs to the power supply circuits 105 a-105 c. The resetsignal Vrs is switched to a high level or a low level. The high level ofthe reset signal corresponds to a steady state and the low levelcorresponds to a reset state.

The host 111 transmits control signals Vcnt1-Vcnt3 described later tothe power supply circuits 105 a-105 c so as to control the power supplycircuits 105 a-105 c and exchanges control signals and informationsignals with various apparatuses (not shown). Referring to the figure,the control signals Vcnt1-Vcnt3 transmitted from the host 111 to thepower supply circuits 105 a-105 c are illustrated using a single signalline. Actually, a plurality of controls signals are transmitted.

The specific circuit structure of the individual soft start circuit 101and the common soft start circuit 103 will be described later (FIG. 9and FIGS. 10A-10D).

(2) Description of the Power Supply Apparatus

A description will be given, with reference to FIGS. 1 through 6C, ofthe operating procedure followed in the power supply apparatus accordingto the embodiment between power-on and power-off.

(2-1) At Power-on

A description will be given, with reference to FIG. 1 and FIG. 2, of theprocedure followed at power-on. At power-on, common soft start isperformed by the common soft start circuit 103 and the common soft startcapacitor 104 for the entire channels.

The host 111 controls on and off of the switches SW at power-on.

The switches SW1 a-SW1 c are closed as illustrated in FIG. 1 and theother switches are open so as to connect soft start terminals 720 a-720c of the power supply circuits 105 a-105 c to the common soft startcircuit 103 and the common soft start capacitor 104. At power-on, thepower supply circuits 105 a-105 c are soft started by the common softstart circuit 103 and the common soft start capacitor 104.

FIGS. 2A through 2C are time charts showing voltage variation atrespective nodes occurring at power-on. FIG. 2A illustrates thevariation of the power supply voltage Vcc. FIG. 2B illustrates thevariation of a common soft start voltage Vsse of the common soft startcircuit 103. FIG. 2C illustrates the variation of the voltages Vo1-Vo3supplied to the loads 117 a-117 c. In FIGS. 2A-3C, the vertical axisrepresents a voltage (V) and the horizontal axis represents time t(msec). At power-on, t=0.

Referring to FIG. 2A, the rise time of the power supply voltage Vcc isT20 (msec). An overshoot is caused without the soft start circuit.Referring to FIG. 2B, the rise time of the common soft start voltageVsse of the common soft start capacitor 104 is T21 (msec).

FIG. 2C illustrates the variation of the voltages Vo1-Vo3 (V) suppliedto the loads 117 a-117 c when soft start is performed by using thecommon soft start circuit 103 and the common soft start capacitor 104.As illustrated in FIG. 2C, the rise time of the supply voltages(Vo1-Vo3) is T22 (msec).

Thus, at power-on, the switches SW1 a, SW1 b and SW1 c are closed sothat the power supply circuits 105 a-105 c are gradually started by thecommon soft start voltage Vsse output from the common soft start circuit103.

(2-2) When Off Signal is Transmitted (Host 111->Power Supply Circuit 105b)

In the power supply apparatus 1000 according to at least an embodiment,power saving is achieved by turning the plurality of power supplycircuits 105 a-105 c on and off individually in accordance with theoperating condition of the loads 117 a-117 c. For this purpose, the host111 transmits the control signals Vcnt1-Vcnt3 for individuallycontrolling the on and off state of the power supply circuits 105 a-105c. In this embodiment, the high level of the control signals Vcnt1-Vcnt3corresponds to an on state and the low level corresponds to an offstate.

A description will be given, by referring to FIG. 3 and FIGS. 4A-4C, ofthe operating procedure for turning off the power supply circuit 105 bwhen the supply of the voltage Vo2 to the load 117 b is no longernecessary.

FIG. 3 is a block diagram illustrating the structure of the power supplyapparatus 1000 according to at least an embodiment of the presentinvention. After common soft start is completed, the reset signal Vrsoutput from the voltage reduction reset circuit 113 is at a high level.When the control signal Vcnt2 transmitted from the host 111 designates ashift of the power supply circuit 105 b to an off state while the resetsignal Vrs remains at a high level, the switches SW1 b-SW3 b connectedto the soft start terminal 720 b of the power supply circuit 105 b areall opened.

FIGS. 4A through 4C are time charts indicating voltage variation atrespective nodes occurring when the power supply circuit 105 b is turnedoff. FIG. 4A illustrates the variation of the control signal Vcnt2transmitted from the host 111 to the power supply circuit 105 b (thehigh level corresponds to an off state and the low level corresponds toan on state). FIG. 4B illustrates the variation of the voltage suppliedto the load 117 b. FIG. 4C illustrates the variation of the voltagessupplied to the load 117 a and the load 117 c. In FIGS. 4A-4C, thevertical axis represents a voltage (V) and the horizontal axisrepresents time t (msec).

Referring to FIG. 4A, the host control signal Vcnt2 transmitted from thehost 111 to the power supply circuit 105 b is switched from a low levelto a high level at time T40, designating a shift to an off state.

The power supply circuit 105 b receiving the instruction to shift to anoff state from the host 111 suspends its voltage step-down operation. Asa result, the voltage Vo2 supplied to the load 117 b is dropped startingat time T40. Voltage supply to the load 117 b is eventually suspended.FIG. 4C illustrates the (constant) voltages Vo1 and Vo13 supplied to theload 117 a and the load 11 c, respectively.

(2-3) Restart of Power Supply Apparatus (Host 111->Power Supply Circuit105 b)

As described, in the power supply apparatus 1000 according to at leastan embodiment of the present invention, the operation of the pluralityof power supply circuits 105 a-105 c is individually turned on or off inaccordance with the operating conditions of the loads 117 a-117 c.Accordingly, in order to restart the power supply circuit 105 afterturning it off, the control signal Vcnt transmitted from the host 111 tothe power supply circuit 105 is switched to a low level.

A description will now be given, by referring to FIG. 3 and FIGS. 5A-5C,of the operating procedure followed when the host 111 directs the powersupply circuit 105 b in an off state to shift to an on state. In thiscase, the power supply circuit 105 b is soft started using theindividual soft start circuit 101.

The power supply circuit 105 b directed by the host 111 to shift to anon state closes only the switch SW3 b (indicated by a broken line 301 inFIG. 3) and maintains the switches SW1 b and the switch SW2 b open. Withthis, the power supply circuit 105 b connects its soft start terminal720 b to the individual soft start circuit 101 via the second terminal1013. Accordingly, the power supply circuit 105 b is soft started atrestart by a soft start voltage Vssi output from the individual softstart circuit 101.

FIGS. 5A through 5C are time charts indicating voltage variation atrespective nodes when the power supply circuit 105 b in an off stateshifts to an on state. FIG. 5A illustrates voltage variation of thecontrol signal Vcnt2 transmitted from the host 111 to the power supplycircuit 105 b (the high level corresponds to off and the low levelcorresponds to on). FIG. 5B illustrates the variation of the individualsoft start voltage Vssi output from the individual soft start circuit101. FIG. 5C illustrates the variation of the voltage Vo2 supplied tothe load 117 b. In FIGS. 5A-5C, the vertical axis represents a voltage(V) and the horizontal axis represents time t (msec).

As illustrated in FIG. 5A, the control signal Vcnt2 transmitted from thehost 111 to the power supply circuit 105 b shifts from a high level to alow level at time T50, designating a shift to an on state. As a result,as illustrated in FIG. 5B, the individual soft start voltage Vssi outputfrom the second terminal 1013 of the individual soft start circuit 101rises to a maximum voltage VH in a period T51 (msec).

FIG. 5C illustrates the variation of the voltage Vo2 (V) supplied to theload 117 b occurring when soft start is performed using the individualsoft start circuit 101. As illustrated, the rise time of the supplyvoltage (Vo2) is T52 (msec).

The power supply circuit 105 b opens the switch SW3 b and closes theswitch SW2 b in a period after time T53, when the individual soft startvoltage Vssi output from the second terminal 1013 of the individual softstart circuit 101 fully rises. Details will be described later. As aresult, the soft start terminal 720 b of the power supply circuit 105 bis connected to the first terminal 1011 of the individual soft startcircuit 101. The soft start terminal 720 b is fixed at the maximumvoltage VH output from the first terminal 1011 of the individual softstart circuit 101.

(2-4) When Power is Turned Off

A description will now be given of the operating procedure followed atpower-off, when power derived from the power supply voltage Vcc, aninput voltage to the power supply circuit 105 b, is turned off.

When power-off is designated, the reset signal Vrs output from thevoltage reduction reset circuit 113 to the power supply circuits 105a-105 c is brought to a low level. When the reset signal Vrs goes low,the power supply circuits 105 a-105 c switch their destination ofconnection from the individual soft start circuit 101 to the common softstart circuit 103 and the common soft start capacitor 104.

As illustrated in FIG. 1, the power supply circuits 105 a-105 c closethe switches SW1 a-SW1 c, respectively, and open the switches SW2 a, SW3a, SW2 b, SW3 b, SW2 c and SW3 c so as to connect the soft startterminals 720 a-720 c of the power supply circuits 105 a-105 c to thecommon soft start circuit 103 and the common soft start capacitor 104.

FIGS. 6A through 6C are time charts showing voltage variation atrespective nodes occurring at power-off.

FIG. 6A illustrates the variation of the power supply voltage Vcc. FIG.6B illustrates voltage variation of the reset signal Vrs (the high levelcorresponds to a steady state and the low level corresponds to a resetstate). FIG. 6C illustrates the variation of the voltage supplied to theloads 117 a-117 c. In FIGS. 6A-6C, the vertical axis represents avoltage (V) and the horizontal axis represents time t (msec).

Referring to FIGS. 6A and 6B, the power supply voltage Vcc starts todrop at time T60. After time T60 elapses, the reset signal Vrstransmitted from the voltage reduction reset circuit 113 to the powersupply circuits 105 a-105 c is brought to a low level. When the resetsignal Vrs goes low, designating a shift to a reset state, the voltagesVo1-Vo3 supplied to the loads 117 a-117 c drop as illustrated in FIG.6C.

(3) Circuit Structure of Power Supply Circuit

FIG. 7 illustrates an example of circuit structure of the power supplycircuit 105 according to at least an embodiment of the presentinvention. The following description concerns the structure of the powersupply circuit 105 b. The description applies similarly to the powersupply circuit 105 a and the power supply circuit 105 c. FIG. 7 alsoillustrates the coil 107 b and the smoothing capacitor 109 b connectedto the power supply circuit 105 b.

The power supply circuit 105 b includes an error amplifier 709, anoscillator 701, a driver 705, a switching transistor 703, a diode 713, aPWM comparator 707, resistors R1 and R2, and a feedback resistor 711.

The soft start terminal 720 b is a terminal for connection with theswitches SW1 b-SW3 b. The soft start voltage Vss output from theindividual soft start circuit 101 or the common soft start circuit 103is input to the terminal. The power supply voltage Vcc is applied to theinput voltage terminal 730. The voltage Vo2 supplied to the load is fedback to a feedback terminal 740.

The switching transistor 703, the diode 713 and the externally coupledcoil 107 and the smoothing capacitor 109 b constitute a switchingregulator of a step-down type. The switching regulator steps down thepower supply voltage Vcc input to the input voltage terminal 730 to atarget value determined by a reference voltage Vref and outputs thestepped-down voltage.

The power supply voltage Vcc, an input voltage, is input to an end ofthe switching transistor 703. The other end of the switching transistor703 is connected to the rectifying diode 713. The voltage at a nodebetween the switching transistor 703 and the diode 713 is output as thesupply voltage Vo2 via the coil 107 b.

A pulse-width modulated switching signal output from the driver 705 isinput to the gate of the switching transistor 703. The switchingtransistor 703 is subject to on and off control in accordance with theduty ratio of the switching signal. The coil 107 b is supplied with acurrent alternately via the switching transistor 703 and the diode 713,in accordance with the on and off of the switching transistor 703. As aresult, energy conversion is performed by the coil 107 b and thesmoothing capacitor 109 b so that the power supply voltage Vcc isstepped down. The coil 107 b and the smoothing capacitor 109 bconstitute a filter and the supply voltage Vo2 is output as a smootheddc voltage.

The supply voltage Vo2 is divided by the resistors R1 and R2 beforebeing fed to the error amplifier 709. The error amplifier 709 amplifiesa voltage error of the supply voltage Vo2 divided by the resistors R1and R2 with respect to the soft start voltage Vss applied to the softstart terminal 720 and a voltage error of the supply voltage Vo2 dividedby the resistors with respect to the reference voltage Vref. The erroramplifier 709 outputs the error voltage Verr to the PWM comparator 707.The feedback resistor 711 is connected to the error amplifier 709.

The oscillator 701 outputs a triangular wave at a predeterminedfrequency that defines a switching period. The resistor 715 a and thecapacitor 715 b are provided to adjust the oscillating frequency of theoscillator 701.

The PWM comparator 707 compares the error voltage Verr output from theerror amplifier 709 with the oscillating voltage Vosc output from theoscillator 701. If Verr>Vosc, the PWM comparator 707 generates ahigh-level pulse-width modulated PWM signal Vpwm. If Verr<Vosc, alow-level PWM signal Vpwm is output.

The driver 705 controls the gate voltage of the switching transistor 703in accordance with the PWM signal Vpwm generated by the PWM comparator707 so as to turn on or off the switching transistor 703.

The power supply circuit 105 b according to at least an embodimentmaintains the voltage Vo2 supplied to the load 117 b according to thePWM scheme for controlling the on-time ratio (the ratio between theconduction time of the switching transistor 703 and the oscillationperiod of the oscillator 701) of the switching transistor 703 (a mainswitching element) by a gate drive pulse. The on-time ratio isdetermined by comparing the output voltage Vosc of the oscillator 701,which is a voltage input to the inverting input of the PWM comparator707, with the error voltage Verr output from the error amplifier 709,which is a voltage input to the non-inverting input of the PWMcomparator 707.

In this embodiment, the soft start function is achieved by graduallyincreasing the on-time ratio by progressively extending a duration inwhich the error voltage Verr is higher than the output voltage Vosc ofthe oscillator 701 (interval T71->interval T72->interval T73) asillustrated in FIG. 8.

Further, since the soft terminal 720 is connected to the switches SW1 b,SW2 b and SW3 b according to this embodiment, one of “the common softstart circuit 103 and the common soft start capacitor 104” and “theindividual soft start circuit 101” is connected to the non-invertinginput of the error amplifier 709, by selectively closing the switchesSW1 b, SW2 b and SW3 b.

As mentioned earlier, according to the embodiment, “the common softstart circuit 103 and the common soft start capacitor 104”, areconnected to the non-inverting input of the error amplifier 709 atpower-on. Therefore, the common soft start voltage Vsse is input to thenon-inverting input as the soft start voltage Vss. The feedback voltageVerr of FIG. 8 is gradually increased with the common soft start VoltageVsse.

Further, according to this embodiment, the individual soft start circuit101 is connected to the non-inverting input of the error amplifier 709when the power supply circuit 105 b shifts from an off state to an onstate. Accordingly, the individual soft start voltage Vssi output fromthe individual soft start circuit 101 is input to the non-invertinginput of the error amplifier 709 as the soft start voltage Vss. Theerror voltage Verr of FIG. 8 is increased with the individual soft startvoltage Vssi.

(4) Circuit Structure of Common Soft Start Circuit

FIG. 9 illustrates an example of circuit structure of the common softstart circuit 103 and the common soft start capacitor 104 according toat least an embodiment.

An end of the common soft start capacitor 104 is grounded.

The other end thereof is connected to a node C1 between the common softstart circuit 103 and the switches SW1 a, SW1 b and SW1 c. A constantcurrent source 801 is for charging the common soft start capacitor 104.The constant current source 801 is connected to the power supply Vcc. Anode between the constant current source 801 and an end of a resistor805 is connected to the common soft start capacitor 104. The other endof the resistor 805 is connected to the drain of a transistor 803. Thesource of the transistor 803 is grounded. The gate of the transistor 803is connected to an electronic control circuit (not shown) and receivesan timing signal.

With this circuit structure, the transistor 803 is turned on or off inaccordance with the timing signal input to the gate of the transistor803. When the transistor 803 is turned on, the common soft startcapacitor 104 is discharged so that the voltage at the output terminal1014 drops to a level close to the ground potential.

When the transistor 803 is turned off, the potential at the outputterminal 1014 is increased as a result of a charge Q being stored in thecommon soft start capacitor 104 at power-on by a constant currentsupplied from the constant current source 801. More specifically, thecoefficient of electrostatic capacity of the common soft start capacitor104 is determined such that∫idt=it=Q=C×Vcc, t=C×Vcc/i=5-10 msec,where the charge Q=C×Vcc (C denotes the coefficient of electrostaticcapacity of the common soft start capacitor 104) and the current=i.

The resistor 805 of the common soft start circuit 103 illustrated inFIG. 9 is not essentially. While the transistor 803 is illustrated inFIG. 9 as a MOS transistor, a bipolar transistor may also be used.

(5) Circuit Structure of Individual Soft Start Circuit

FIGS. 10A-10D illustrate circuit structures of the individual soft startcircuit 101 according to at least an embodiment. In any of the types ofillustrated individual soft start circuit 101, there are provided afirst terminal 1011 and a second terminal 1013. The individual softstart voltage Vssi that is gradually increased with time is output fromthe second terminal 1013. The maximum value VH of the individual softstart voltage Vssi is constantly output from the first terminal 1011.

(5-1) With Regard to Circuit Structure of FIG. 10A

A DAC circuit 900 is provided with a digital input terminal 909 a, ananalog output terminal 904 a, a maximum voltage terminal 903 a and aminimum voltage terminal 905 a. The maximum voltage terminal 903 a isconnected to a latch power supply 910 so that the voltage VH is appliedto the maximum voltage terminal 903 a. The minimum voltage terminal 905a is grounded.

The digital input terminal 909 a is connected to a timing generationcircuit (not shown). The timing generation circuit is controlled by acontrol signal from the host 111. The first terminal 1011 outputs themaximum voltage VH output from the latch power supply 910.

In accordance with a control signal input to the digital input terminal909 a, the analog output terminal 904 a outputs a voltage with a rampwaveform with a minimum voltage VL, which is applied to the minimumvoltage terminal 905 a, and a maximum voltage VH, which is applied tothe maximum voltage terminal 903 a.

The voltage output from the analog output terminal 904 a is output fromthe second terminal 1013.

With this circuit structure, the potential at the second terminal 1013is increased in accordance with an increase in digital code input to thedigital input terminal 909 a. When the potential reaches the maximumvalue (VH), the power supply circuits 105 a-105 c connect themselveswith the latch power supply 910 by closing the switch SW2 a (or SW2 b,SW2 c) and opening the switch SW3 a (or SW3 b, SW3 c). With this, thesoft start voltages Vss1-Vss3 input to the respective soft startterminals 720 a-720 c can be fixed.

(5-2) With Regard to Circuit Structure of FIG. 10B

The first terminal 1011 outputs the voltage VH output from the latchpower supply 910. An end of an individual soft start capacitor 920 isgrounded and the other end thereof is connected to a constant currentsource 922. The voltage at a node between the capacitor 920 and thesource 922 is output from the second terminal 1013. The constant currentsource 922 charges the individual soft start capacitor 920. A nodebetween the constant current source 922 and the individual soft startcapacitor 920 is connected to the collector of a transistor 902. Theemitter of the transistor 902 is grounded. The base of the transistor902 is connected to a timing generation circuit (not shown). The timinggeneration circuit is driven by a control signal from the host 111.

With this circuit, when the control signal Vcnt transmitted from thehost 111 designates a shift to an on state, the transistor 902 is turnedoff, causing the individual soft start capacitor 920 to be charged bythe constant current supplied from the constant current source 922. As aresult, the potential at the second terminal 1013 is increased. When thepotential reaches the maximum value VH, the power supply circuits 105a-105 c connect themselves with the latch power supply 910 by closingthe switch SW2 a (or SW2 b, SW2 c) and opening the switch SW3 a (or SW3b, SW3 c). With this, the soft start voltages Vss1-Vss3 input to therespective soft start terminals 720 a-720 c can be fixed.

(5-3) With Regard to Circuit Structure of FIG. 10C

The structure of the individual soft start circuit 101 of FIG. 10C issuch that the constant current source 922 of FIG. 10B is replaced by aresistor 924. The first terminal 1011 outputs the voltage VH output fromthe latch power supply 910. A node between the resistor 924 and thelatch power supply 910 is connected to the first terminal 1011. A nodebetween the resistor 924 and the individual soft start capacitor 920 isconnected to the collector of the transistor 902. The emitter of thetransistor 902 is grounded. The base of the transistor 902 is connectedto a timing generation circuit (not shown). The timing generationcircuit is driven by a control signal from the host 111.

With this circuit, when the control signal Vcnt transmitted from thehost 111 designates a shift to an on state, the transistor 902 is turnedoff, causing the individual soft start capacitor 920 to be charged. As aresult, the potential at the second terminal 1013 is increased inaccordance with a time constant determined by the resistance of theresistor 924 and the capacitance of the individual soft start capacitor920. When the potential reaches the maximum value VH, the power supplycircuits 105 a-105 c connect themselves with the latch power supply 910by closing the switch SW2 a (or SW2 b, SW2 c) and opening the switch SW3a (or SW3 b, SW3 c). With this, the soft start voltages Vss1-Vss3 inputto the respective soft start terminals 720 a-720 c can be fixed.

(5-4) With Regard to Circuit Structure of FIG. 10D

The structure of the individual soft start circuit 101 of FIG. 10D issuch that the constant current source 922 of FIG. 10B is replaced by atransistor 926. The first terminal loll outputs the voltage VH outputfrom the latch power supply 910. An end of the individual soft startcapacitor 920 is grounded and the other end thereof is connected to thesecond terminal 1013.

The gate of the transistor 926 is connected to a timing generationcircuit (not shown). The timing generation circuit transmits a PWMsignal in accordance with a control signal from the host 111 so as tochange the conduction time of the transistor 926. As the conduction timevaries, the transistor 926 functions as a variable resistor.

A node between the drain of the transistor 926 and the latch powersupply 910 is connected to the first terminal 1011. A node between thesource of the transistor 926 and the individual soft start terminal 920is connected to the collector of the transistor 902 and the secondterminal 1013. The emitter of the transistor 902 is grounded. The baseof the transistor 902 is connected to a timing generation circuit (notshown). The timing generation circuit is driven by a control signal fromthe host 111.

With this circuit, when the control signal Vcnt transmitted from thehost 111 designates a shift to an on state, the transistor 902 is turnedoff, causing the individual soft start capacitor 920 to be charged. As aresult, the potential at the second terminal 1013 is increased. When thepotential reaches the maximum value VH, the power supply circuits 105a-105 c connect themselves with the latch power supply 910 by closingthe switch SW2 a (or SW2 b, SW2 c) and opening the switch SW3 a (or SW3b, SW3 c). With this, the soft start voltages Vss1-Vss3 input to therespective soft start terminals 720 a-720 c can be fixed. While thetransistor 902 is illustrated in FIGS. 10A, 10B and 10C as a bipolartransistor, a MOS transistor may also be used.

(6) Signal Waveform and Voltage Variation when Host Transmits ControlSignal

FIGS. 11A through 11I are time charts showing signal waveforms andvoltage variation occurring when the control signal is transmitted fromthe host. The following description with reference to FIGS. 11A-11Irelates to a case where the DAC circuit of FIG. 10A is used in theindividual soft start circuit 101.

FIG. 11A shows a waveform of the control signal Vcnt1 supplied to thepower supply circuit 105 a. The high level corresponds to an off signaland the low level corresponds to an on signal.

FIG. 11B shows the waveform of the control signal Vcnt2 supplied to thepower supply circuit 105 b. The high level corresponds to an off signaland the low level corresponds to an on signal.

FIG. 11C shows the waveform of the output voltage from the DAC circuit900, i.e., the individual soft start voltage Vssi output from the secondterminal 1013 of the individual soft start circuit 101.

FIGS. 11D-11G show the waveforms indicating the open and closedconditions of the switch SW3 a, the switch SW2 a, the switch SW3 b andthe switch SW2 b, respectively. The high level corresponds to a closedstate and the low level corresponds to an open state.

FIG. 11H shows the waveform of the soft start voltage Vss1 input to thesoft start terminal 720 a of the power supply circuit 105 a.

FIG. 11I shows the waveform of the soft start voltage Vss2 input to thesoft start terminal 720 b of the power supply circuit 105 b.

(6-1) With Regard to Operation Between T101 and T103

The power supply circuit 105 a, receiving the control signal Vcnt1 at alow level from the host 111, opens the switch SW1 a and the switch SW2 aand closes the switch SW3 a so as to connect itself to the secondterminal 1013 of the individual soft start circuit 101. Accordingly, thesoft start voltage Vss1 input to the soft start terminal 720 a of thepower supply circuit 105 a is increased (FIG. 11H) as the individualsoft start voltage Vssi output from the DAC circuit 900 is increased(FIG. 11C).

(6-2) With Regard to Operation Between T103 and T105

The output voltage from the DAC circuit 900 (FIG. 11C) reaches themaximum value VH at time T103, triggering the power supply circuit 105 ato close the switch SW2 a. Thereafter, the switch SW2 a and the switchSW3 a are both closed until time T105. This is to prevent the voltageoutput from the individual soft start circuit 101 from becomingintermittent. In this state, the switch SW1 a is open. In this period,the individual soft start circuit 101 and the power supply circuit 105 aare connected to each other via the first terminal 1011 and the secondterminal 1013. As illustrated in FIG. 11H, the soft start voltage Vss1input to the soft start terminal 720 a is maintained at the maximumvalue VH.

(6-3) With Regard to Operation Between T105 and T107

At time T105, i.e., when a predetermined period T120 elapses since timeT103, the power supply circuit 105 a opens the switch SW3 a. Thereafter,only the switch SW2 a is closed until the control signal Vcnt1 makes atransition from a low level to a high level. Accordingly, the individualsoft start circuit 101 and the power supply circuit 105 a are connectedto each other via the first terminal 1011 after time T105 and until thecontrol signal Vcnt1 makes a transition to a high level. As illustratedin FIG. 11H, the soft start voltage Vss1 input to the soft startterminal 720 is maintained at the maximum value VH.

(6-4) With Regard to Operation Between T107 and T109

When the control signal Vcnt2 transmitted from the host 111 is switchedto a low level, the power supply circuit 105 b opens the switch SW1 band the switch SW2 b and closes the switch SW3 b so as to connect itselfto the second terminal 1013 of the individual soft start circuit 101.Accordingly, as the individual soft start voltage Vssi output from theDAC circuit 900 is increased (FIG. 11C), the soft start voltage Vss2input to the soft start terminal 720 b of the power supply circuit 105 bis increased (FIG. 11I).

(6-5) With Regard to Operation Between T109 and T111

The soft start voltage Vssi reaches the maximum value VH at time T109,triggering the power supply circuit 105 b to close the switch SW2 b.Thereafter, the switch SW2 b and the switch SW3 b are both closed untiltime T111. This is to prevent the voltage output from the individualsoft start circuit 101 from becoming intermittent. In this state, theswitch SW1 b is open. In this period, the individual soft start circuit101 and the power supply circuit 105 a are connected to each other viathe first terminal 1011 and the second terminal 1013. As illustrated inFIG. 11H, the soft start voltage Vss2 input to the soft start terminal720 b is maintained at the maximum value VH.

(6-6) With Regard to Operation Between T111 and T113

At time T111, i.e., when a predetermined period T120 elapses since timeT109, the power supply circuit 105 b opens the switch SW3 b. Thereafter,only the switch SW2 b is closed until time T113. Accordingly, theindividual soft start circuit 101 and the power supply circuit 105 b areconnected to each other via the first terminal 1011 in a period betweenT111 and T113. The soft start voltage Vss2 input to the soft startterminal 720 b is maintained at the maximum value VH.

(6-7) With Regard to Operation after T113

The control signal Vcnt2 makes a transition to a high level at timeT113, triggering the power supply circuit 105 b to open the switch SW2 bagain. In this state, the switches SW1 b-SW3 b connected to the powersupply circuit 105 b are all opened so that the soft start voltage Vss2input to the soft start terminal 720 converges to 0V.

Described above is an explanation based on the embodiment. Those skilledin the art will envision many other possible variations and improvementswithin the scope of the present invention.

For example, while the circuit illustrated in FIG. 7 is used as thepower supply circuit and the circuit illustrated in FIG. 9 is used asthe common soft start circuit, alternative circuit structures arepossible.

The power supply circuit of the embodiment is described as beingimplemented by a switching regulator of a step-down type, a switchingregulator of a step-up type may also be used.

The power supply apparatus according to at least an embodiment of theinvention may be comprehended as follows.

The power supply apparatus comprises: a plurality of power supplycircuits which supply voltages to respective loads; a common soft startswitch connected to each of the plurality of power supply circuits; anindividual soft start switch connected to each of the plurality of powersupply circuits; a common soft start circuit commonly connected to theplurality of power supply circuits via the common soft start switch; andan individual soft start circuit commonly connected to the plurality ofpower supply circuits via the individual soft start switch.

The power supply circuit may connect itself to the individual soft startcircuit in response to a control signal, by closing the individual softstart switch connected to the circuit.

The power supply circuit may connect itself to the common soft startcircuit and the common soft start capacitor at power-off, by closing thecommon soft start switch connected to the circuit.

The common soft start circuit preferably comprises: a constant currentsource for charging the common soft start capacitor via a node connectedto the common soft start capacitor; and a transistor for discharging thecommon soft start capacitor via the node in accordance with a timingsignal input to the transistor gate.

The individual soft start circuit preferably comprises: first and secondnodes connected to the individual soft start switch; a DAC circuit foroutputting a voltage of a ramp waveform via the first node; and a latchpower supply for outputting a predetermined voltage via the second node.

The individual soft start circuit in one variation preferably comprises:first and second nodes connected to the individual soft start switch; anindividual soft start capacitor; a constant current source for chargingthe individual soft start capacitor; a second transistor for dischargingthe individual soft start capacitor via a node between the individualsoft start capacitor and the constant current source and via the firstnode, in accordance with a timing signal input to the transistor gate;and a latch power supply for outputting a constant voltage via thesecond node.

The individual soft start circuit in another variation preferablycomprises: first and second nodes connected to the individual soft startswitch; an individual soft start capacitor; a resistor for charging theindividual soft start capacitor; a second transistor for discharging theindividual soft start capacitor via a node between the individual softstart capacitor and the resistor and via the first node, in accordancewith a timing signal input to the transistor gate; and a latch powersupply for outputting a constant voltage via the second node.

The individual soft start circuit in another variation preferablycomprises: first and second nodes connected to the individual soft startswitch; an individual soft start capacitor; a first transistor forcharging the individual soft start capacitor; a second transistor fordischarging the individual soft start capacitor via a node between theindividual soft start capacitor and the first transistor and via thefirst node, in accordance with a timing signal input to the transistorgate; and a latch power supply for outputting a constant voltage via thesecond node.

The apparatus according to at least an embodiment of the presentinvention may be used to drive a variety of loads.

While the preferred embodiments of the present invention have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

1. A power supply apparatus comprising: a plurality of power supplycircuits which supply voltages to respective loads; an individual softstart circuit including a first terminal for outputting a predeterminedfixed voltage and a second terminal outputting a voltage that varieswith time, the individual soft start circuit being operated when any ofthe plurality of supply voltages output from the plurality of powersupply circuits is caused to make a transition individually; anindividual soft start switch provided for each of the plurality of powersupply circuits, the individual soft start switch connecting the powersupply circuit with the first terminal or the second terminal of theindividual soft start circuit, wherein the plurality of power supplycircuits apply soft start control to the respective supply voltages inaccordance with the soft start voltage output from the individual softstart circuit.
 2. The power supply apparatus according to claim 1,wherein the plurality of power supply circuits close the individual softstart switch in accordance with an externally supplied control signaland apply soft start control to the respective supply voltages inaccordance with the soft start voltage output from the individual softstart circuit.
 3. The power supply apparatus according to claim 2,wherein the individual soft start circuit comprises: a latch powersupply which outputs a predetermined voltage; and a digital-to-analogconversion circuit which outputs a voltage of a ramp waveform inaccordance with a digital signal input to a digital input terminal, theoutput voltage at its maximum level being equal to the predeterminedvoltage, wherein the predetermined voltage is output from a firstterminal and the voltage with a ramp waveform is output from a secondterminal.
 4. The power supply apparatus according to claim 2, whereinthe individual soft start circuit comprises: a latch power supply whichoutputs a predetermined voltage; an individual soft start capacitor; aconstant current source which charges the individual soft startcapacitor by supplying a constant current thereto; and a transistorwhich is provided parallel with the individual soft start capacitor anddischarges the individual soft start capacitor in accordance with atiming signal input to the gate of the transistor, wherein thepredetermined voltage is output from a first terminal and a voltage at anode between the individual soft start capacitor and the constantcurrent source is output from a second terminal.
 5. The power supplyapparatus according to claim 2, wherein the individual soft startcircuit comprises: a latch power supply which outputs a predeterminedvoltage; an individual soft start capacitor; a resistor provided betweenthe individual soft start capacitor and the latch power supply; and atransistor which is provided parallel with the individual soft startcapacitor and discharges the individual soft start capacitor inaccordance with a timing signal input to the gate of the transistor,wherein the predetermined voltage is output from a first terminal and avoltage at a node between the individual soft start capacitor and theresistor is output from a second terminal.
 6. The power supply apparatusaccording to claim 2, wherein the individual soft start circuitcomprises: a latch power supply which outputs a predetermined voltage;an individual soft start capacitor; a charge transistor provided betweenthe individual soft start capacitor and the latch power supply; and adischarge transistor which is provided parallel with the individual softstart capacitor and discharges the individual soft start capacitor inaccordance with a timing signal input to the gate of the dischargetransistor, wherein the predetermined voltage is output from a firstterminal and a voltage at a node between the individual soft startcapacitor and the discharge transistor is output from a second terminal.7. The power supply apparatus according to claim 1, wherein theindividual soft start circuit comprises: a latch power supply whichoutputs a predetermined voltage; and a digital-to-analog conversioncircuit which outputs a voltage of a ramp waveform in accordance with adigital signal input to a digital input terminal, the output voltage atits maximum level being equal to the predetermined voltage, wherein thepredetermined voltage is output from a first terminal and the voltagewith a ramp waveform is output from a second terminal.
 8. The powersupply apparatus according to claim 1, wherein the individual soft startcircuit comprises: a latch power supply which outputs a predeterminedvoltage; an individual soft start capacitor; a constant current sourcewhich charges the individual soft start capacitor by supplying aconstant current thereto; and a transistor which is provided parallelwith the individual soft start capacitor and discharges the individualsoft start capacitor in accordance with a timing signal input to thegate of the transistor, wherein the predetermined voltage is output froma first terminal and a voltage at a node between the individual softstart capacitor and the constant current source is output from a secondterminal.
 9. The power supply apparatus according to claim 1, whereinthe individual soft start circuit comprises: a latch power supply whichoutputs a predetermined voltage; an individual soft start capacitor; aresistor provided between the individual soft start capacitor and thelatch power supply; and a transistor which is provided parallel with theindividual soft start capacitor and discharges the individual soft startcapacitor in accordance with a timing signal input to the gate of thetransistor, wherein the predetermined voltage is output from a firstterminal and a voltage at a node between the individual soft startcapacitor and the resistor is output from a second terminal.
 10. Thepower supply apparatus according to claim 1, wherein the individual softstart circuit comprises: a latch power supply which outputs apredetermined voltage; an individual soft start capacitor; a chargetransistor provided between the individual soft start capacitor and thelatch power supply; and a discharge transistor which is providedparallel with the individual soft start capacitor and discharges theindividual soft start capacitor in accordance with a timing signal inputto the gate of the discharge transistor, wherein the predeterminedvoltage is output from a first terminal and a voltage at a node betweenthe individual soft start capacitor and the discharge transistor isoutput from a second terminal.